Demand for semiconductor memories has increased with expanding global markets for electronic devices, such as digital cameras, camcorders, MP3 players, digital multimedia broadcasting (DMB) receivers, navigation systems and mobile phones. In addition, there has been an increasing demand for high-capacity memories that are driven at high speed and reduced power consumption in terms of performance characteristics as compared to conventional memories. Under such circumstances, considerable research efforts have been made in developing next-generation memories that include the advantages and features inherent to dynamic random access memories (DRAMs), static random access memories (SRAMs) and flash memories. Phase-change RAMs (PRAMs), magnetoresistive RAMs (MRAMs), ferroelectric RAMs (FeRAMs) and polymer memories are currently considered next-generation memories. Of these, PRAMs possess the advantages of conventional highly integrated DRAMs, high-speed SRAMs and non-volatile NAND flash memories, and have excellent characteristics in terms of compatibility with conventional integration processes of complementary metal-oxide-semiconductor (C-MOS) field effect transistors (FETs). Based on these advantages, PRAMs have attracted more and more attention because of the greatest possibility of successful commercialization.
Since a paper reported by S. Lai (Intel) and T. Lowrey (Ovonyx) at the International Electronic Device Meeting (IEDM) in 2001, extensive research and development have been conducted on phase-change RAMs (PRAMs). Phase-change RAMs are non-volatile memories that use materials capable of inducing a reversible phase change between crystalline (low electrical resistance) and amorphous (high electrical resistance) phases due to Joule heating generated in response to an applied current or voltage to write data.
Metal alloys and chalcogenides are currently used as representative phase-change materials of PRAMs. Particularly, the composition of GexSbyTez (GST), a chalcogenide, is now being investigated.
In CMP processes for phase-change materials of PRAM devices that are currently being developed, silicon oxide (SiO2) is used to form polish stop layers.
Polishing uniformity and surface imperfections (e.g., dishing and erosion) during polishing of patterned wafers are greatly affected by some processing factors, e.g., polishing and etch rates on phase-change materials, polishing uniformity of silicon oxide films and polishing selectivity between phase-change materials and silicon oxide films.
On the other hand, slurries for polishing aluminum, copper, tungsten and other metal wires are mainly employed in semiconductor manufacturing processes. Since these metal layer materials are composed of a single element, unlike phase-change materials of PRAM devices, they induce no phase change. Therefore, the conventional metal materials cannot be used for PRAM devices and cause a significant difference in layer characteristics.
Depending on the choice of an oxidizing agent, an abrasive, and other useful additives, a CMP slurry for polishing metal wires can be tailored to provide effective polishing on metal layers at desired polishing rates while minimizing surface imperfections, defects, corrosion, and erosion. Furthermore, the polishing slurry may be used to provide controlled polishing selectivities to other thin-film materials, such as titanium, titanium nitride, tantalum, tantalum nitride, oxides and the like.
Unlike conventional metal layers composed of a single element, such as copper (Cu) or tungsten (W), layers of phase-change memory devices to be polished are composed of advanced materials consisting of particular elements, such as sulfur (S), selenium (Se), germanium (Ge), antimony (Sb), tellurium (Te), silver (Ag), indium (In), tin (Sn), gallium (Ga), and the like, in a specified ratio to undergo a reversible phase change between crystalline and amorphous phases. Since the characteristics of the materials to be polished are different from those of conventional metal layer materials, there exists a strong need to develop novel polishing compositions.
Ideal slurry compositions for polishing phase-change materials of phase-change memories (PRAMs) should meet the following requirements: i) the phase-change materials must be etched and polished at high rates; ii) the polishing selectivity between the phase-change materials and polish stop layers must be high; iii) dishing, erosion, pattern non-uniformity, imperfections (e.g., scratches, defects and corrosion), and the like, must be minimized; and iv) there must be no change in the composition and phase of elements constituting the surface of the phase-change materials after polishing.